1. Field of the Invention
This invention relates generally to sound reproduction systems, and more particularly to an improved feedback system which compensates for the nonlinear characteristics of a signal-to-sound transducer, such as a loudspeaker.
2. Description of the Related Art
In the field of sound reproduction it is well known that the sound level produced by conventional loudspeakers diminishes near the limits of human hearing. For instance, at low frequencies, the mechanical and electrical characteristics of a loudspeaker tend to reduce the sound level output by the loudspeaker. This is primarily caused by the current limiting effects of the series resistance inherent in the speaker's drive coil at low frequencies.
There have been many attempts to compensate for these undesirable characteristics so that constant sound output from a loudspeaker can be achieved over the entire range of human hearing. These attempts have been made because the response of the human auditory system is not constant and varies with the frequency and intensity of sound waves. These inefficiencies of the human auditory system and the need for an appropriate compensation system for a loudspeaker are discussed in greater detail in U.S. Pat. No. 3,449,518 issued June 10, 1969 to Erath.
One method for compensating the low frequency inefficiencies of a loudspeaker is described in the above-mentioned patent. The patent discloses a degenerative feedback network which attempts to maintain constant level of sound output from a loudspeaker. The loudspeaker is driven by a broad-band audio amplifier, and the circuit elements of the feedback network are tuned to match the low frequency response of the loudspeaker. The feedback network receives a current signal from the voice coil of the loudspeaker and delivers a degenerative feedback signal to an input of the broad-band audio amplifier. Since the degenerative feedback signal is "tuned" to cancel the undesirable response of the voice coil, the low frequency response of the loudspeaker is improved.
One embodiment of the feedback network includes an inductor and a capacitor which are selected to be equivalent to the fundamental resonance of the speaker cone. The network further includes a resistor which is selected to represent the lumped mechanical resistance in the loudspeaker and an inductor that is selected to be equivalent to the leakage inductance of the voice coil In other words, the frequency compensation network is selected to be equivalent to the impedance of the loudspeaker throughout the frequency range of the loudspeaker.
In theory the frequency compensation network should function quite well and produce a constant sound output level from the loudspeaker over its entire frequency range. In practice, however, this constant output level could not be achieved. This is primarily due to the non-ideal characteristics of the circuit elements of the frequency compensation network. For instance, inductors have some finite resistive component which interferes with the theoretical ideal characteristics of the feedback network. Therefore, the frequency compensating feedback network disclosed in the previously mentioned patent, while being an improvement in the art, does not fully correct the problem.
Another attempt to correct the low frequency inefficiencies of a loudspeaker is disclosed in U.S. Pat. No. 4,335,274 issued June 15, 1982 to Ayers. To overcome basic defects in the low frequency response of a loudspeaker, two degenerative feedback circuits are provided which attempt to alleviate an impedance peak and an impedance valley in the low frequency range of the loudspeaker. A first feedback circuit applies degenerative feedback, proportional to the current flowing through the drive coil, to an audio amplifier; and a second feedback circuit applies degenerative feedback, proportional to voltage induced in feedback coil which is disposed about the voice coil of the loudspeaker, to the audio amplifier. However, one problem with this type of compensation system is that the speaker must be modified to accept the feedback coil. Another problem exists because reactive elements are used in the feedback circuits, and these reactive elements include non-ideal characteristics as mentioned previously.
Another method of compensating for the low frequency response characteristics of a loudspeaker utilizes a transducer to sense the sound pressure level output by the loudspeaker. In response to the sound pressure level, a feedback signal, proportional to the sound output level of the loudspeaker, is applied to an associated audio amplifier. While this does raise the low frequency response of a loudspeaker, it does not necessarily provide a constant sound output level. Moreover, the transducers themselves have limited frequency response characteristics, and, therefore, cannot fully overcome the poor low frequency response characteristics of the associated loudspeaker.